DOC. 60 379 Doc. 60 ON THE DEVELOPMENT OF OUR VIEWS CONCERNING THE NATURE AND CONSTITUTION OF RADIATION by A. Einstein [Deutsche Physikalische Gesellschaft, Verhandlungen 7 (1909): 482-500. Also in Physikalische Zeitschrift 10 (1909): 817-826] (Presented at the session of the Division of Physics of the 81st Meeting of German Scientists and Physicians in Salzburg on September 21, 1909.) [1] (Cf. above p. 417) Once it had been recognized that light exhibits the phenomena of inter- ference and diffraction, it seemed hardly doubtful any longer that light is to be conceived as a wave motion. Since light can also propagate through vacuum, one had to imagine that vacuum, too, contains some special kind of matter that mediates the propagation of light waves. For the interpretation of the laws of the propagation of light in ponderable bodies, it was necessary to assume that this matter, which was called luminiferous ether, is present in them too, and that in the interior of ponderable bodies as well, it is essentially the luminiferous ether that mediates the propagation of light. The existence of this luminiferous ether seemed beyond doubt. The first volume of the excel- lent textbook by Chwolson, which was published in 1902, contains in the Introduction the following sentence about the ether: "The probability of the hypothesis on the existence of this agent borders extraordinarily closely on certainty." [2] However, today we must regard the ether hypothesis as an obsolete stand- point. It is even undeniable that there is an extensive group of facts concerning radiation that shows that light possesses certain fundamental properties that can be understood far more readily from the standpoint of Newton's emission theory of light than from the standpoint of the wave theory. [3] It is therefore my opinion that the next stage in the development of theore- tical physics will bring us a theory of light that can be understood as a kind of fusion of the wave and emission theories of light. To give reasons for this opinion and to show that a profound change in our views on the nature and constitution of light is imperative is the purpose of the following remarks.